<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <h2 class="counter">0</h2>
  <button class="increment">+1</button>
  <button class="decrement">-1</button>

  <script>
    // 1.获取所有元素
    const counterEl = document.querySelector(".counter");
    const incrementEl = document.querySelector(".increment");
    const decrementEl = document.querySelector(".decrement");

    // 2.定义变量
    let counter = 0;
    counterEl.innerHTML = counter;

    // 3.监听按钮点击
    incrementEl.addEventListener('click', () => {
      counter += 1;
      counterEl.innerHTML = counter;
    })
    decrementEl.addEventListener('click', () => {
      counter -= 1;
      counterEl.innerHTML = counter;
    })
  </script>
</body>

</html>